1. Field of the Invention
This invention relates to an exhaust gas cleaning system for an internal combustion engine, and more particularly to an exhaust gas cleaning system which traps and burns combustible particulates contained in exhaust gas of, for instance, a diesel engine.
2. Description of the Prior Art
There have been known various exhaust gas cleaning systems. In the system disclosed in Japanese Unexamined Patent Publication No. 56(1981)-98519, a catalytic filter for trapping and oxidizing combustible particulates in the exhaust gas is provided in the exhaust system of an engine, and an auxiliary fuel injection system for injecting liquid or gaseous auxiliary fuel is provided in the exhaust system upstream of the catalytic filter, whereby the exhaust gas temperature is raised above the ignition point of the combustible particulates by oxidization under catalytic action of the catalytic filter and combustion promoting effect of the auxiliary fuel so that the combustible particulates are burnt and removed from the catalytic filter to refresh the filter.
However, the conventional system is disadvantageous in that since the exhaust gas temperature is relatively low (200.degree. to 300.degree. C. in the case of diesel engines ) when the engine is in the steady operating range, the exhaust gas cannot be heated above the ignition point of the combustible particulates regardless of the combustion promoting effect of the auxiliary fuel, and accordingly the combustible particulates cannot be sufficiently burnt. When combustion of the trapped combustible particulates is unsatisfactory, the catalytic filter cannot be sufficiently refreshed and clogging of the filter will occur so that the back pressure of the engine is increased to the point that normal operation of the engine is prevented.
As disclosed in our Japanese patent application No. 59(1984)-64627, the ignition point of the combustion particulates can be lowered so that the combustible particulates can be positively burnt even during the steady operation of the engine in which the exhaust gas temperature is relatively low by providing a particulate filter having only particulates-trapping function but no catalytic effect in the exhaust system instead of the catalytic filter and by injecting into the exhaust passage upstream of the particulate filter a material for promoting combustion of the combustible particulates. The combustion promoting material may generally comprise an emulsion of a mixture of a catalytic component and a hydrocarbon component. By injecting the combustion promoting material into the exhaust system upstream of the particulate filter, the catalytic component and the hydrocarbon component uniformly adhere to the surface of the combustible particulates trapped by the particulate filter. This substantially lowers the ignition point of the combustible particulates. However, this technique has been found to be disadvantageous in that when the combustion promoting material is injected during the high speed operation of the engine in which the amount of exhaust gas is large, the injection flow of the combustion promoting material is disturbed by the high-speed exhaust gas and the combustion promoting material does not uniformly adhere to the surface of the combustible particulates over the entire area thereof.
Further, when the amount of exhaust gas is large, the exhaust gas temperature is generally high and the combustion temperature is apt to become abnormally high. As a result, cracks are liable to be produced in the filter.